1. Field of the Invention
The present invention relates to an information processing apparatus of a recording/reproducing apparatus for writing or reading information to/from a medium, which faces a plurality of probes by a physical interaction, a scanning tunneling microscope (STM), or the like and, more particularly, to a control circuit of a plurality of probes.
2. Related Background Art
In recent years, a scanning tunneling microscope (hereinafter, abbreviated to STM) which can directly observe an electron structure of surface atoms of a conductor has been developed (refer to G. Binnig et al., "Phys Rev Lett" Vol. 49, Nol, p. 57-61, 1982) A real space image can be measured at a high resolution irrespective of a monocrystal or an amorphous material.
The STM uses a phenomenon such that when a voltage is applied between a metal probe (probe electrode) and a conductive material and the probe is allowed to approach up to a distance of about 1 nm, a tunnel current flows. The tunnel current is very sensitive to a change in distance between the probe and the material. By scanning the probe so as to keep the tunnel current constant, various kinds of information regarding a whole electron cloud in the real space can also be read. In this instance, a resolution in the in-plane direction is equal to about 0.1 nm.
Therefore, by applying the principle of the STM, high density recording or reproduction can be sufficiently executed on the order of atoms (sub nanometer). For example, in a recording/reproducing apparatus disclosed in Japanese Patent Laid-open Application No. 61-80536, atomic particles adsorbed on the medium surface are removed by an electron beam or the like and data is written and the data is reproduced by the STM.
There has also been proposed a method whereby a thin film layer made of a material having a memory effect for the switching characteristics of a voltage current, for example, a .pi. electron system organic compound or chalcogen compound class is used as a recording layer and the recording or reproducing operation is executed by the STM (refer to Japanese Patent Laid-open Application No. 63-161552, Japanese Patent Laid-open Application No. 63-161553). According to such a method, now assuming that a size of recording bit is equal to 10 nm, the recording or reproduction of a large capacity of 1012 bits/cm.sup.2 can be performed. Further, an apparatus such that a plurality of probes are formed on a semiconductor substrate and a displacement is caused in a recording medium which faces those probes and data is recorded has been proposed for the purpose of miniaturization (Japanese Patent Laid-open Application No. 1-196751). For example, a multiprobe head in which 2500 probes are arranged on a silicon chip of a square size of 1 cm.sup.2 in a matrix form of (50.times.50) probes and the above material having the memory effect are combined, so that digital data of 400 Mbits per probe, a total recording capacity of 1 Tbits, can be recorded or reproduced.
In this instance, a method whereby each probe is formed in a cantilever shape having a length of about hundreds of .mu.m and is driven is considered. Hitherto, as a method of forming such a cantilever, there is a method whereby a semiconductor process is applied and by using a working technique for performing a fine working process onto one substrate, a cantilever having a multilayer structure such as a thin piezoelectric film, metal film, or the like is formed. (Refer to T. R. Albrecht et al., "Microfabrication of Integrated Scanning Tunneling Microscope", Proceedings of 4th International Conference on Scanning Tunneling Microscope/Spectroscopy, 1990.)
Such an information processing apparatus using the tunnel current or field radiation current has a function to keep the distance between the probe and the sample constant. A signal processing circuit of the information processing apparatus having such a function has been described in, for example, "Nikkei Microdevice", Vol. November, pages 81 to 97, 1986. In such a signal processing circuit, by using a fact that the tunnel current can be expressed by the following equation
I=EXP(-K.multidot.z) PA1 I: tunnel current PA1 z: distance between the probe and the sample PA1 K: constant PA1 (1) When executing the information process such as recording, reproduction, or the like, it is necessary to precisely control the distance (in the Z direction) between each probe of the multiprobe head and the medium which faces it. For this purpose, there is considered a method whereby, for example, a control system for performing the Z direction control of the distance between the probe and the medium is provided for every probe and a control section comprising each control system is provided on the outside of the probe head. In such a case, although the multiprobe head can be constructed in a small size, there are problems such that the control section which is provided on the outside of the apparatus is increased in size by a volume corresponding to the number of probes and it is difficult to realize a small information processing apparatus which advantageously uses a feature of the STM. PA1 (2) In the case of forming a plurality of probes by a fine working process such as an IC process or the like, the occurrence of a variation in the working process cannot be avoided. Such a variation in working process, for example, causes a variation in displacement sensitivity of an actuator such as a piezoelectric cantilever or the like or a variation in sensitivity of the probe tip, namely, a variation in work function value, so that it makes it difficult to control a plurality of probes. In a surface observing apparatus .such as an STM or the like, a variation in change in brightness of a surface concave/convex signal of each probe is caused. In the recording/reproducing apparatus, a variation in amplitude of a reproduction signal is caused for every probe, so that a reproduction error occurs.
where,
the detected tunnel current is logarithm-converted and supplied to a servo circuit, and the interval between the probe and the sample is controlled on the basis of an output of the servo circuit such as to keep the tunnel current constant. In the above equation, the constant value K is a value to which a work function of the tip of the probe and the sample is reflected. It is known that the value of the work function of the STM probe changes in dependence on a contamination or the like of the probe. (Refer to "Journal of Microscopy", Vol. 152, Ptl, pages 129 to 135, 1988.)
There are, however, the following problems when the multiprobe head having a plurality of probes and the medium which faces it are actually combined and an information process such as recording, reproduction, or are like the executed.